Since there seems to be a lot of Toyota Tacoma owners on here... I've got a 2001 Tacoma DoubleCab with the 3.4L V6 with 117k miles. About 15 months ago the check engine light comes on. No driveability issues. I mention it to a buddy here at work with the same truck and he bets it is the O2 sensor, but cautions me to only use the Denso brand and not a Bosch as he says the Bosch will continue to throw error codes for some reason. I'm skeptical.
Anyway, I get the code read at Adavance Auto and they verify it's the O2 sensor, but they didn't have the Denso one in stock - only the Bosch. I end up buying the Bosch and installing it. Reset the computer and everything is fine for about a month and then the CEL comes on again. No driveability issues. I assume it is the Bosch O2 sensor and just keep resetting the computer every month or two to clear the CEL light because it annoys me just like Sheldon on "Big Bang Theory".
Fast forward to about 4 months ago. I got the local Toyota dealer to change the timing belt (I know - but it was cheap). I asked them to "read the codes" and make sure everything was OK relative to my occasional CEL issue and they did and said it was fine - no issues. Not long after that, the CEL came on again, but now I had some driveability issues. Mainly a loss of power at highway speeds, some mild pinging and a drop of about 4 mpg in gas mileage (I check gas mileage with every tank of gas). If I reset the CEL, everything is fine for a day or two and then it comes back.
I take the truck and get the codes read again and the guy says "O2 sensor". I ask him what it actually says and he shows it to me and it says, "System too lean - bank one". I asked him how he knows that means "O2 sensor" and he said that's what it always is.
I do some checking on the interwebs and come to find out that code is typically the MAF sensor or a vacuum leak. So, this past weekend I remove and clean the MAF sensor and check the entire truck over for vacuum leaks and find nothing.
I put the whole thing back together and it runs better than it has in years until this morning (three days later) and the CEL comes on again.
Anyone have any ideas?